1. Field of the Invention
The present invention relates to a light reflecting material for reflectors or reflection plate materials used for back light systems of various visual display units such as liquid crystal displays, illumination signboards, lighting equipment and the like.
In particular, the present invention relates to a light reflecting material of high reflectance useful as a reflector of so-called edge light type lighting panels, which comprise a light conducting plate and a light source arranged facing at least one side edge of the light conducting plate with the reflector being provided around the light source so as to surround the light source and efficiently cause light from the light source to enter the light conducting plate, or as a light reflecting material efficiently reflecting light from the light conducting plate to a liquid crystal panel.
2. Related Art
As conventional light reflecting materials, there can be mentioned aluminum plates such as those described in Japanese Unexamined Application Publication No. Sho 62-286019, metallic reflection plates such as those described in Japanese Utility Model Unexamined Application Publication No. Hei 4-22755, aluminum plates painted white such as those described in Japanese Unexamined Application Publication No. Sho 63-2002, films painted white such as those described in Japanese Unexamined Application Publication No. Hei 6-67174, foamed white films such as those described in Japanese Unexamined Application Publication No. Hei 3-256090 and the like.
In particular, backlights of the edge light type now in wide use often use silver deposited films of high reflectance as a reflector to obtain high brightness in lighting panels.
As a light source for backlights of the edge light type, tubular lamps including hot-cathode tubes and cold-cathode tubes are generally used. However, problems may arise when a silver deposited film is used as the reflector of these lamps. For example, oscillation of the light source and lighting failure may occur because conductive silver is present very close to the light source. Furthermore, silver deposited films are very expensive.
On the other hand, lighting failure could be obviated by using a foamed white film for the reflector. As disclosed in Japanese Unexamined Application Publication No. Hei 3-256090, however, the flexibility required for good handling during wrapping about the light source can be ensured only up to a maximum thickness of about 75 .mu.m when the foamed white film is a foamed white polyester film.
However, if such a thin foamed white film is used as the reflector, it may disadvantageously leak light from the light source to the outside and hence lower the brightness of the lighting panel.
In the case of aluminum plates and films painted white, good hiding property can be obtained if titanium dioxide is used as a white pigment. However, titanium dioxide absorbs light of 450 nm and shorter wavelengths and the light reflectance is therefore disadvantageously suppressed below a certain level.
Further, if barium sulfate is used as a white pigment, the layer containing the barium sulfate must have a large thickness or contain a large amount of added barium sulfate to obtain high reflectance because barium sulfate has low hiding power. If the thickness of the layer containing barium sulfate is increased to obtain sufficient hiding power, it becomes difficult to wrap the film around the lamp and hence the efficiency of the operation is degraded. If thickness of the layer is made smaller to obtain sufficient flexibility, the quantity of barium sulfate added has to be increased beyond the critical pigment concentration in order to obtain sufficient hiding power and the coated layer is therefore liable to suffer cohesive failure.
The object of the present invention is to solve the problems of the conventional materials mentioned above, i.e., to provide a light reflecting material which is free from the problems of oscillation of light source, lighting failure and the like, exhibits high reflectance, and has sufficient flexibility enabling easy wrapping around a light source.